1. Field
The present invention generally relates to a radio frequency (RF) coil of a magnetic resonance imaging (MRI) system. More particularly, the present invention relates to a multi-channel phased array coil for obtaining an emphasis image of cerebral cortex of a human brain in a high magnetic field MRI system.
2. Background
A magnetic resonance imaging (MRI) system is widely used in the field of medical diagnosis due to its capability of providing 3-dimensional and/or high-resolution images without harming a human body. In particular, a high magnetic field (e.g., 7 Tesla) MRI system is attracting attention in the field since it can provide images with higher signal to noise ratio (SNR) and higher resolution compared to a low magnetic field (e.g., 1.5 Tesla or 3 Tesla) MRI system. Further, the high magnetic field (7 Tesla) MRI system can provide images of cerebral cortex, thereby making it possible to provide better medical services to patients with brain diseases.
A radio frequency (RF) coil of an MRI system is generally used to form RF field to excite spins of a region of interest (“ROI”) (i.e., a region whose images are intended to be acquired) and to detect the variation of the magnetic field from the spins, which precess due to the RF field. To obtain images of high quality in the MRI system, the RF coil should meet the following two requirements. First, for transmission purposes, the coil should be able to form a homogeneous magnetic filed at the ROI to excite protons of nuclei. Second, for reception purposes, the coil should be able to acquire RF signals with the same gain at the ROI.
In a conventional MRI system, a coil for both transmission and reception is used to obtain images of a human brain. In the low magnetic field MRI system, the magnetic field generated by the RF coil, which is exclusively used for a human brain, is homogeneous over the wide range. However, in the high magnetic field MRI system with the frequency of 300 MHz or above, it is impossible for the conventional coil for both transmission and reception to form a homogeneous magnetic field inside the human brain. This is accounted by the phenomenon in which the effective RF wavelength is shortened due to the permittivity of the human brain positioned at the interior of the RF coil. The shortened wavelength results in standing waves inside the human brain. As a result, the periphery of the image becomes dark while the center part of the image is bright, which makes the diagnosis difficult.
Furthermore, it is difficult to develop RF coils used in the high magnetic field MRI system compared to those used in the low magnetic field MRI system. As discussed below, there are several reasons for such difficulty.
First, the wavelength of the signals in the high magnetic field MRI system is shortened. This is because the resonance frequency is proportional to the magnitude of the magnetic field. In case of low frequency AC signals, the signal wavelength is not a concern since it is considerably longer than the size of the circuits or lines. However, when the frequency is higher than 300 MHz, the wavelength is shortened to the order of centimeters. Thus, a phase superposition occurs on the lines. In the case of low-frequency signal with long wavelength, the waveform distortion is negligible in spite of the superposition of the AC waveforms on the lines because the phase shift is slight. However, in case of high-frequency signal with short wavelength, the signal waveform may experience severe distortion with the result that the original waveform may not be maintained due to the superposition with different phases. In order to avoid such a problem, the size of the circuit should be reduced. Further, all dimensions of the circuit including its length and size should be downsized when designing the coils.
Second, a crosstalk increases as the frequency increases. Even in the case of low-frequency signals, the interference (i.e., crosstalk) between lines exists. However, as the frequency is higher, even the non-substantial part of the coil (i.e., the part which does not conduct transmitting or receiving signals) acts as the substantial part of the coil, thus radiating more electromagnetic energy. As a result, the coupling at the RF circuit becomes more important. That is, along with the wavelength problem, the design of RF coil circuit becomes more like a structural design. Further, there are many cases of circuit designing in the RF coils using such a coupling.
Third, a high antenna gain should be provided. Since the signals transmitted and received by the RF coil are wireless signals in most cases, the coil should have a small size due to the specification requited in real use. However, the smaller the size of the coil is, the smaller the gain is. Accordingly, more sophisticated technologies should be integrated to provide a higher gain with smaller size when designing the RF coil.
Fourth, the effect of the external noise should be considered. The resonator has a predetermined bandwidth centering around the center frequency. Theoretically, to obtain the maximum signal to noise ratio (SNR) when designing the resonator, the reflection coefficient is designed to approach infinity at the desired frequency. Also, it is designed to be like noise at other frequencies. However, it is practically impossible to realize such a circuit. Thus, the resonator is inevitably affected by the external noise. Furthermore, since the bandwidth becomes wider as the frequency is further increased (e.g., if the bandwidth corresponds to 1% of the frequency, the bandwidth is 3 MHz at the frequency of 300 MHz used in the high magnetic field MRI system, while the bandwidth is 100 kHz at the frequency of 10 MHz), the effect of the external noise increases. Thus, the SNR of the overall circuit decreases. Although such a problem relates to the frequency rather than the design of the RF coil structure, it is one of the most important factors to be considered when designing the resonator. Although using a narrow bandwidth at a high frequency makes it possible to obtain a high SNR, it is very difficult to implement such a circuit which can use a narrow bandwidth with respect to the frequency. Even if such a circuit is implemented, it may be sensitive to a variation of the external conditions.
In order to address and resolve the above-mentioned problems, the present invention provides a radio frequency (RF) coil with high signal to noise ratio (SNR) and high resolution to emphasize the cerebral cortex of a human brain in a high magnetic field (7 Tesla) magnetic resonance imaging (MRI) system.